1. Field of the Invention
The invention relates to the field of spectrometers. In particular the invention relates to Fourier-transform infrared (FTIR) spectrometers operating in the mid-wave IR (MWIR) and long-wave IR (LWIR) range.
2. Description of the Related Technology
A large number of applications exist for FTIR spectrometers operating in the MWIR and LWIR wavelength regions. The MWIR region encompasses wavelengths of about 2-7 μm and the LWIR region encompasses wavelengths of about 8-15 μm. These infrared regions provide distinguishing signatures for many organic and inorganic materials, and are sometimes known as the “fingerprint regions”. These fingerprint regions can be useful in a variety of applications including analytical chemistry, biochemistry, and materials research. Additional applications may include environmental sensing, chemical bio-sensing, and condition-based maintenance. Currently, most FTIR spectrometer applications are laboratory based due to their large size and complexity. Small, compact FTIR spectrometers would be an enabling technology for many commercial and military applications that require field use of FTIR spectrometers.
Fiber FTIR spectrometers have been demonstrated in the near-IR at wavelengths of about 0.8 μm and 1.5 μm using single mode silica fiber and either a semiconductor laser at 0.8 μm, or an Er-doped silica fiber amplified spontaneous emission (“ASE”) at wavelengths of 1.53 μm to 1.58 μm, as a source. Extension of the useful wavelengths into the mid-wave IR covering the 2-15 μm wavelength range, where FTIR spectrometers have their greatest applicability, appears to be impossible in silica due to the transmission window of silica fiber, which cuts off at around 2 μm. No MWIR or LWIR broadband sources exist that can be coupled with sufficient efficiency into a single-mode fiber to enable FTIR spectroscopy in these wavelength regions.
Some methods and devices have been disclosed in the past for performing spectroscopy in the mid-wave and long-wave IR region. Some of these methods and devices are discussed below.
U.S. Pat. No. 6,230,044 to Afanassieva et al. discloses fiber optic evanescent wave FTIR spectroscopy using fiber optic sensors operated in an attenuated total reflection regime in the middle infrared region. The useful wavelengths can be extended into the near infrared or the far infrared regions. The device utilizes spherical mirrors or focusing lenses.
U.S. Pat. No. 6,157,856 to Sanghera et al. discloses a FTIR spectrometer using a clad chalcogenide glass fiber core. The chalcogenide glass can transmit over a range of about 2-12 μm. A nichrome wire light source is also employed. U.S. Pat. No. 5,841,546 to Carangelo et al. also discloses a FTIR spectrometer having an optical fiber structure that includes a fiber core constructed of chalcogenide glass.
U.S. Pat. No. 5,754,715 to Melling discloses a fiber optic spectroscopic probe that is used with a FUR spectrometer. The probe is made of fiber optic bundles. The optical fibers are made from IR-transmitting materials, one of which is chalcogenide glass. The fiber optics can operate in the mid-IR range.
U.S. Pat. No. 5,739,536 to Bucholtz et al. discloses a FTIR and an infrared transmitting chalcogenide optical fiber for receiving and analyzing reflected radiation. An IR source is used. Middle infrared radiation is preferably employed in this device.
Although the above patents disclose various arrangements for performing spectroscopy, they do not utilize components that enable the provision of compact and cost efficient FTIR spectrometers.
Therefore, there exists a need for methods and apparatus for performing FTIR spectroscopy in the MWIR and LWIR regions that enable the provision of compact and cost-efficient FTIR spectrometers. Accordingly, it is an object of certain embodiments of the invention to provide a method and apparatus for performing spectroscopy in the MWIR and LWIR regions.